Hydrocarbon fuel reformers are well known, especially in the fuel cell arts wherein a hydrogen-rich reformate is generated from a hydrocarbon input fuel for consumption as a fuel by a downstream fuel cell system comprising, for example, a solid oxide fuel cell (SOFC) stack. A prior art reformer typically includes a reforming bed formed of a porous foam material having a washcoat applied to the surfaces of the foam and a precious-metal catalyst applied to the washcoat. The hydrocarbon fuel, when passed through the reforming bed in the presence of oxygen and/or steam, becomes oxidized to molecular hydrogen and carbon monoxide, both of which are excellent fuels for an SOFC stack.
Reformers are divided generally into two categories: exothermic and endothermic. In exothermic reforming, the only oxidant typically is oxygen derived from air, and the exothermic oxidation reaction provides sufficient thermal energy to maintain the reaction at the elevated temperatures required for reforming. In endothermic reforming, the oxidant typically is water vapor but may also include oxygen from air. Because the overall reaction is endothermic, an outside source of heat must be provided to the reformer, which, in the case of a reformer coupled to an SOFC, typically is accomplished by burning either anode stack exhaust gas which is still rich in hydrogen or the flow of original hydrocarbon fuel.
In either case, reforming cannot commence until the catalyst is raised to a temperature typically greater than about 550° C.; thus, a reformer/SOFC fuel cell system cannot begin generating electricity until both the reformer and the fuel cell stack are at elevated temperature.
It is known in the art of reformer start-up to provide to the reformer a combustible is fuel/air mixture of the hydrocarbon fuel to be reformed and to ignite the flowing mixture for several seconds, passing the hot combustion gases through the reformer to heat the catalyst. In earlier reformers (particularly exothermic reformers), the combustion gases were passed directly through the catalytic bed before ignition was terminated and reforming started; however, in more recent reformers, the combustion gases are passed around the catalyst bed which is contained in tubes that separate the bed from the gases. Fuel/air mixture can pass through the catalyst bed while the temperature is being increased to the reforming temperature. And, indeed, in endothermic reforming continuous exogenous heating of the catalyst bed may be necessary.
A well-known challenge in the reformer art is how to promote both rapid and efficient heat transfer from the hot combustion gases to the catalyst bed. Inefficient transfer causes longer heating times at start-up, which is wasteful of fuel and dissatisfies the fuel cell user. Further, inefficient transfer increases the heat lost to the system in the exhaust gas and thus is also wasteful of fuel.
U.S. Pat. No. 6,759,016 B2 discloses to dispose a plurality of catalyst tubes within a plurality of burner tubes to define a narrow annular gap therebetween, and to pass the burner combustion gases through the annular gap to heat the catalyst tubes. Although this arrangement provides a relatively high surface-to-volume ratio for the combustion gases to the catalyst tubes, heat transfer rate is still limited by the available surface area of the catalyst tubes.
U.S. Pat. No. 5,876,469 discloses to arrange a porous media composed of ceramic foams around the catalyst tubes. Flue gas is passed through the porous media which is heated thereby and transfers heat to the catalyst tubes via re-radiation and conduction. A problem with this arrangement is that conduction is the most efficient means of heat transfer but requires a conductive path, and the porous media is only fractionally in conductive contact with the catalyst tubes. Further, a foam structure has an inherently long conductive path for heat migration and therefore is effectively an insulator, in an application that requires just the opposite. Further, even in open cell foams, the tortuous pathway for gas passing through creates an objectionably large pressure drop for combustion gases.
What is needed in the art is an improved multiple-tube catalytic reformer wherein the reforming domains are separate from the combustive heating domains, and wherein the materials and construction of each domain are optimized for the different functions to be performed.
It is a principal object of the present invention to improve the effectiveness and reduce the cost of manufacturing of a CPOx catalytic hydrocarbon reformer.